Generally, a refrigeration system includes a compressor, a condenser, an expansion valve, and an evaporator. Refrigerant vapor is compressed to a high pressure by the compressor and is conducted through the condenser where it is cooled to form a liquid under high pressure. This high pressure liquid is then adiabatically expanded through the expansion valve into the evaporator. In the evaporator, the refrigerant absorbs heat from the surroundings of the evaporator, which transforms the low pressure liquid refrigerant into a vapor. In this process, the environment surrounding the evaporator, for example, a refrigerator case, is cooled. The refrigerant vapor is then returned to the compressor via a suction line.
Generally, it is desirable to control the amount of liquid refrigerant returning to the inlet of the compressor from the evaporator. In some cases, liquid refrigerant may dilute the lubricating oil in a typical hermetic compressor and thus cause damage to the compressor. Also, liquid refrigerant may damage certain of the compressor components, such as the compressor reed valves.
Another concern with many refrigeration systems is the presence of ice on the evaporator coils. During normal operation of many refrigeration systems, the evaporators may operate at temperatures low enough for water vapor to crystallize on the evaporator coils. This can produce a “frost” on the coils, which may reduce the efficiency of the refrigeration system and may result in liquid refrigerant flooding the compressor. As a result, the surfaces of the evaporator coils must periodically be defrosted.
Various techniques for defrosting refrigeration systems are known. For example, one method for defrosting refrigeration systems is to reverse the refrigeration cycle. When the refrigeration cycle is reversed, hot refrigerant vapor from the compressor is directed into the evaporator outlet, through the evaporator, into the condenser inlet, through the condenser, and back into the compressor. A problem with this method is that often the temperature of refrigerant entering the compressor is so low that some liquid is introduced into the compressor. As discussed above, the presence of liquid in the compressor may damage or destroy the compressor. In addition, the temperature of the refrigerant entering the evaporator may be too low for rapid or complete defrosting of the evaporator. Thus, the defrost cycle may be very time consuming or the evaporator may not be completely defrosted.
As such, there is a need for improved refrigeration systems, in particular, for refrigeration systems in which the amount of liquid refrigerant entering the compressor is controlled and/or in which the amount of ice build up on the evaporator coils is controlled.